Increasing demand for low power consumption in the PC space is driving customers to conserve power in all operating states. This is especially true in the hibernate S4 or S5 state where a laptop appears to the user to be off. Several Manufacturers have also defined a new state called Connected Standby or “InstantGo” that requires ultra-lower power operation. In either state, the USB subsystem is very limited in how much power can be consumed. In the case of a hub, it should consume less than ˜1 mW. That power number is unreachable in the conventional hubs. When off, the hub cannot perform a battery charging handshake, so the system effectively loses support for standard defined battery charging 1.2 or for device specific charging in the “off” state—even if power is still available on the Vbus pin of the USB port.
A second concern is overall power budget allotted to the USB port charging. 4 ports of charging support could theoretically approach 10 A. Most systems cannot support such a high current budget. A more manageable limit might be 5 A—which 2 ports could theoretically consume.